Regulation of innate immunity in tilapia: activation of nonspecific cytotoxic cells by cytokine-like factors

Abstract

Exposure of tilapia ( Oreochromis niloticus) to water temperatures of 10–15°C for 3–5 min produces physiological stress responses characterized by immediate phenotypic and immunological changes. In the present study, this general stress response was utilized as a model system to study innate immunity mediated by soluble factors and cytotoxic cells. Acute innate cytotoxic responses of nonspecific cytotoxic cells (NCC) in the peripheral blood (PBL), anterior kidney (AK) and spleen (SPL) were measured. Following temperature stress, the levels of NCC activity depended on the presence of soluble factors and on the cell compartments from which the NCC were obtained. NCC from PBL of stressed tilapia had 30× or greater cytotoxic activity compared to nonstressed PBLs from controls. NCC activity from the AK and SPL of stressed tilapia was lower than controls. Flow cytometric analysis of NCC in each tissue showed that increased cytotoxicity was not produced by increased numbers of NCC. To determine the mechanism of amplification of cytotoxicity, NCC from nonstressed tilapia were passively treated with serum from temperature stressed tilapia. Serum containing the “stress activated serum factor” (SASF) passively increased naive NCC cytotoxicity (from PBL) 3–4 fold. The cytotoxic cell response was inhibited by addition of anti-NCC monoclonal antibody 5C6. These data indicated that NCC are (at least one of) the target cells for SASF. SASF required only 15 min pre-incubation with naive NCC to activate cytotoxicity. Activation was nonreversible and concentration dependent. Pretreatment of NCC with SASF reduced the assay time required to amplify target cell cytotoxicity from 12–24 h to 6 h. SASF amplification of NCC cytotoxicity was not restricted by different histological types of target cells. Determination of select physical/chemical properties of SASF revealed: complete heat inactivation of cytotoxicity amplification following 55°C and 65°C pretreatment; SASF was thermostable at room temperature to 45°C for 15 min; and freeze-thaw treatment reduced but did not completely remove amplification activity. The molecular weight range of SASF activity was identified in a 50–100 kDa fraction obtained by differential dialysis. SASF appears to be a protein sensitive to trypsin digestion.